Creep Versus Consolidation in Tunnelling through Squeezing Ground—Part B: Transferability of Experience.

This paper investigates potential differences between creep and consolidation in mechanised tunnelling through squeezing ground, placing focus on the practical question of using experiences gained from existing tunnels about the required thrust force as a reference for tunnels of different diameter or adjacent tunnels. The investigations focus on two aspects. First, the effect of the tunnel diameter on the risk of shield jamming is examined. The paper demonstrates that larger-diameter tunnels are more favourable in poor-quality ground, while the opposite holds in better-quality ground, as well as in the case of pronouncedly time-dependent ground behaviour due to consolidation or creep. Second, the effect of a tunnel on the required thrust force in a neighbouring tunnel built later is examined. The paper shows that this interaction effect is particularly important in water-bearing ground of low permeability, where the drainage action of the first tunnel induces pore pressure relief and ground consolidation in an extensive area, leading to a remarkable reduction of the thrust force in the second tunnel. Conversely, in the case of creep the interaction is negligible even under extremely squeezing conditions, due to the fundamentally different nature of the purely mechanical rheological processes from coupled hydromechanical processes. The presented investigations into the transferability of experiences are valuable for tunnelling practice, in cases of twin tunnels as well as in situations where a smaller-diameter tunnel is constructed prior to the main tunnel (e.g. a pilot tunnel for exploration, advance drainage or ground improvement), or also the opposite (e.g. upgrade of a road tunnel by later construction of a safety tunnel with a smaller diameter). Highlights: The rate of ground deformations is independent of diameter in the case of creep, but extremely increases with decreasing diameter in the case of consolidation. The risk of shield jamming is lower in larger-diameter tunnels crossing poor-quality ground without pronounced time-dependent behaviour. The risk of shield jamming is lower in smaller-diameter tunnels crossing ground of higher quality or exhibiting pronounced time-dependent behaviour. The effect of adjacent tunnel on shield jamming risk is generally minor in the case of creep, even for extreme squeezing conditions and large overcuts. The effect of adjacent tunnel on shield jamming risk is remarkable in the case of consolidation, leading to a substantial reduction of the thrust force. [ABSTRACT FROM AUTHOR]